1. Field of the Invention
The invention relates to magnetic tape cores and somewhat more particularly to a process of producing Ni-Fe magnetic tape cores by annealing and magnetic tempering.
2. Prior Art
It is known to subject tape cores composed of Ni-Fe alloys having 48 to 67 wt. % Ni therein, to a final annealing for about 5 hours at a temperature ranging between 1150.degree. to 1200.degree. C. in pure hydrogen and thereafter to subject the so-annealed tape cores to tempering in a magnetic field which is applied in the plane of the tape parallel to the direction in which the tape material is rolled. By tempering in such a so-called longitudinal magnetic field, one obtains rectangularly-shaped hysteresis loops or, particularly, high dynamic permeabilities upon small modulations [see, for example, Zeitschrift fur Metallkunde (Journal For Metalography), Vol. 57 (1966) pages 240-244].
However, tape cores treated in the above-described prior art process are not suitable for applications which require a large induction rise and a large impulse permeability, i.e., such as in choke coils having a constant pre-magnetization field or in pulse transformers which function in unipolar fashion. For such and similar applications or uses, heretofore available tape cores composed of Ni-Fe-Mo alloys having a relatively large nickel content of 61 to 67 wt. % or, respectively, 74 to 84 wt. % have been used. Such tape cores, after a final annealing for a number of hours at temperatures ranging between 950.degree. to 1220.degree. C. were subjected to a tempering in a magnetic field whose lines of force in the tape core were perpendicular to the direction of the magnetic flux in the cores during actual usage, i.e., perpendicular to the rolling direction of the cores. So-treated tape cores, which are characterized by very flat hysteresis loops, have relatively high impulse permeabilities which, as a function of induction rise, at first exhibit a substantially constant course but upon discharge into saturation, corresponding to induction rises between about 0.4 to 0.8T, quickly decrease to small values far below 4000 (for example, see German Letters Pat. Nos. 1,558,818 and 1,558,820, which respectively correspond to U.S. Pat. Nos. 3,546,031 and 3,556,876).